[dwheeler]

There are many specific causes, time stamps probably being the most common issue. You can see a list of common issues here:

https://reproducible-builds.org/docs/

The main overall issue is that developers don't test to ensure they reproduce. Once it's part of the release tests it tends to stay reproducible.

[acaloiar]

I agree, although I wouldn't describe the overall issue as developers not testing to ensure reproducibility. The reason most builds aren't reproducible is that build reproducibility isn't a goal for most projects.

It would be great if 100% of builds were reproducible, but I don't believe developers shouldn't be testing for reproducibility unless it's a defined goal.

As generalized reproducible build tooling (guix, nix, etc.) becomes more mainstream, I imagine we'll see more reproducible builds as adoption grows and reproducibility is no longer something developers have to "check for", but simply rely upon from their tooling.

[zimbatm]

It's also because the cost of making things reproducible is still too high.

We have the tooling, but it still takes a bit of effort from the developer's side to integrate those into their CI pipeline.

Eventually we will get to a place where this will be the default. It will be integrated into day-to-day tooling like `cargo release`, `npm publish`, ...

[acaloiar]

Typo: I don't believe developers shouldn't be -> I don't believe developers should be